As RFID tags become increasingly ubiquitous in their use in conjunction with the growing range of applications, it is increasingly common for such RFID tags to be used to record a data transaction event between the RFID tag and an appropriate read/write device. By recording the data transaction events a data log of the transactions that includes, amongst other data items, the dates and times of the data transactions can be created that can be used as evidence of the completion of the data transactions. Examples of uses of RFID tags in this manner may involve a portable read/write device being used to complete data transactions between multiple RFID tags, each of which are fixed to separate items, the read/write device maintaining a data log of those items that have been physically visited by the read/write operator such that it can be later verified that all necessary items have been physically visited. For example, this arrangement may be used to verify that all fire extinguishers equipped with an RFID tag have been physically examined by a fire safety officer equipped with a suitable read/write device, alternatively a similar arrangement could be used to verify that a service engineer has physically visited (and thus presumably checked) the appropriate RFID tag equipped machinery, the data log providing the basis for proof of completion of paid for service tasks. In these latter examples, it is advantageous for the read/write device operator not to be able to falsify the data transaction between the read/write device and the respective RFID tags so as to prevent fraudulent work claims being made. For example, it is desirable that it is not possible to falsify the date and/or times at which a data transaction between the read/write device and an RFID tag has been made.